Beware of the handy online tire calculators. They are acurate for passenger tires but not necessarily for off-road tires which are taller than same-size passenger tires.

After putting new, oversized BFG ATs on fresh new American Racing chromed alloys I made my way to Moab for ZuZoo. Before leaving, I checked the online tire calculator which told that the speedo would read 3% slow. As I was cutting across the NE corner of Oregon a very nice state cop used laser to show me the inacuracy of the online calculator. After months of letter-writing to the court in LaGrand, the very nice judge changed the charge from speeding, to "faulty equipment" but it still cost me something over $100.

Borrow or buy a GPS, then you'll know for sure!